Role of gas in supporting grand spiral structure

TL;DR

This paper demonstrates that including gas in the density wave theory of spiral galaxies extends the longevity of spiral patterns and is essential for stability at observed pattern speeds.

Contribution

It introduces a coupled gas-star density wave model, showing gas inclusion prolongs spiral structure lifetime and stabilizes wave patterns.

Findings

Gas inclusion slows group transport, extending spiral pattern lifespan to several billion years.

Adding gas is crucial for stable wave patterns at observed galactic pattern speeds.

The model aligns with observed features and improves understanding of spiral structure persistence.

Abstract

The density wave theory for the grand-design two-armed spiral pattern in galaxies is successful in explaining several observed features. However, the long-term persistence of this spiral structure is a serious problem since the group transport would destroy it within about a billion years as shown in a classic paper by Toomre. In this paper we include the low velocity dispersion component, namely gas, on an equal footing with stars in the formulation of the density wave theory, and obtain the dispersion relation for this coupled system. We show that the inclusion of gas makes the group transport slower by a factor of few, thus allowing the pattern to persist longer - for several billion years. Though still less than the Hubble time, this helps in making the spiral structure more long-lived. Further we show that addition of gas is essential to get a stable wave for the observed pattern speed for the Galaxy, which otherwise is not possible for a one-component stellar disc.